During Release 14, the LTE standard has been extended with support of device-to-device D2D (specified as “sidelink”) features targeting both commercial and Public Safety applications. Some applications enabled by Rel-12 LTE are device discovery, where devices can sense the proximity of another device and associated application by broadcasting and detecting discovery messages that carry device and application identities. Another application consists of direct communication based on physical channels terminated directly between devices.
In Release 14, the LTE specification is extended to include support of vehicle-to-anything V2X communication, which includes any combination of direct communication between vehicles V2V, between vehicles and pedestrians V2P, and between vehicles and infrastructure V2I. V2X communication may take advantage of a network NW infrastructure, when available, but at least basic V2X connectivity should be possible even in case of lack of NW coverage.
FIG. 1 is a schematic diagram illustrating V2X scenarios for an LTE-based Radio Access Network NW. As shown in FIG. 1, V2I (vehicle-to-infrastructure) communications may be provided between a vehicle and the radio access network (RAN), V2V (vehicle-to-vehicle) communications may be provided directly between different vehicles (without communicating through the radio access network), and V2P (vehicle-to-pedestrian) communications may be provided directly between a vehicle and a device held/carried by the pedestrian (e.g., a smartphone, a tablet computer, etc.). V2X communications are meant to include any/all of V2I, V2P, and V2V communications. As used herein, the term wireless terminal UE may refer to any non-network terminal such as a vehicular wireless terminal (e.g., UE-1, UE-2, UE-3, etc.) or a personal wireless terminal held/carried by a pedestrian (e.g., a smartphone, a tablet computer, etc.), and a sidelink communication may refer to wireless communication that is directly terminated between any two such wireless terminals UEs (without passing through network infrastructure. Such sidelink communications may thus include D2D communications, V2V communications, V2P communications, etc.
V2X communications may carry both non-safety and safety information, where each of the applications and services may be associated with specific requirements sets, e.g., in terms of latency, reliability, capacity, etc.
ETSI has defined two types of messages for road safety: Co-operative Awareness Message (CAM) and Decentralized Environmental Notification Message (DENM).
CAM: The CAM message is intended to enable vehicles, including emergency vehicles, to notify their presence and other relevant parameters in a broadcast fashion. Such messages target other vehicles, pedestrians, and infrastructure, and are handled by their applications. CAM message also serve as active assistance to safety driving for normal traffic. The availability of a CAM message is indicatively checked for every 100 ms, yielding a maximum detection latency requirement of less than or equal to 100 ms for most messages. However, the latency requirement for Pre-crash sensing warning may be 50 ms.
DENM: The DENM message is event-triggered, such as by braking, and the availability of a DENM message is also checked for every 100 ms, and the requirement of maximum latency is less than or equal to 100 ms.
The package size of CAM and DENM message varies from 100+ to 800+ bytes and the typical size is around 300 bytes. The message is supposed to be detected by all vehicles in proximity.
The SAE (Society of the Automotive Engineers) also defined the Basic Safety Message (BSM) for dedicated short-range communications (DSRC) with various message sizes defined.
According to the importance and urgency of the messages, the BSMs may be further classified into different priorities.
The Release 14 specification includes two scheduling modes in V2X, hereinafter referred to as Mode 3 and Mode 4 which are further discussed below:                In Mode 3, the eNB schedules the PC5 transmissions (i.e., V2X sidelink transmissions using a PC5 carrier). To schedule a PC5 transmission, the eNB sends a PC5 scheduling grant using the Downlink LTE Uu interface. This grant may include, among other things, information about the time-frequency resources to be used for PC5 transmission (i.e., the specific subframe and RBs). The information about the time of the transmission is relative. One example illustrating this is that if the scheduling grant is received in subframe ‘n’, the PC5 transmission takes place in subframe ‘n’+4.        In Mode 4, a UE schedules its own transmissions according to a distributed resource allocation algorithm that is part of the specification.This disclosure is concerned with Mode 3 scheduling, i.e. the eNB scheduling mode.        
To establish successful communication, the transmitter and receiver should share common time and frequency references. That is, the transmitter and receiver should be synchronized in time and/or frequency. This disclosure is concerned with time synchronization. Time synchronization includes determining the subframe boundaries, i.e., the point in time in which each subframe starts and ends, as well as the index of the subframe.
For cellular communication, i.e., uplink and/or downlink communications between a wireless terminal or UE and a network node in LTE, the eNB network node is the synchronization source. Wireless terminals (or UEs) acquire synchronization through specific signals known as synchronization signals transmitted by the eNB. Two types of cellular synchronization may be distinguished as follows:                Downlink (DL) timing refers to the reference used by the UE to process the signals received from the eNB. This corresponds to the time at which the signal is transmitted from the eNB plus the propagation time between eNB and UE.        Uplink (UL) timing is the reference used by the UE when transmitting signals to the eNB. The UL timing is configured by the eNB using Timing Advance (TA) commands. These commands instruct the UE to move forward or backward relative to the reference by a certain quantity.        
For direct device-to-device (D2D) communication, also referred to as sidelink communications, several synchronization sources are defined in the specification in addition to DL timing:                UTC (Universal Coordinated Time) timing, which is also referred to as GNSS (Global Navigation Satellite Network) timing, refers to an absolute time reference. A UE or eNB can acquire UTC from a GNSS signal, for example.        SLSS (SideLink Synchronization Signal) timing is a timing reference that is transmitted from UE to UE using a distributed time protocol. SLSS timing may originate from the NW, UTC, or some other source and is propagated by the UEs to provide synchronization to UEs that have neither NW nor GNSS coverage.        
Mode-3 deployments for V2X sidelink communications may involve two carriers (a network carrier and a sidelink carrier) as follows:                A sidelink carrier for UE-UE communications may use the LTE PC5 interface, referred to as a PC5 carrier. Sidelink communications directly terminated between two UEs may be transmitted using a sidelink or a PC5 carrier.        A network carrier for eNB-UE communications may use the LTE Uu interface, referred as a Uu carrier. Scheduling grants transmitted from the eNB to the UE for PC5 transmissions may be transmitted using a network or a Uu carrier.        
The PC5 carrier and the Uu carrier could be the same or different carriers. If the carriers are different, then their timing references may be different. That is, at any node (UE, eNB) the subframe number in the PC5 and Uu carriers may not be the same. Similarly, the subframe boundaries in the PC5 and Uu carriers may not be the same. Depending on the case, the actual relationship between the two timings may or may not be known by the different nodes. Due to hardware inaccuracies and other considerations outside the scope of this disclosure, a timing reference may drift over time. Consequently, the relationship between the two timings may vary over time.
Accordingly, a wireless terminal may transmit sidelink transmissions over a sidelink carrier based on scheduling information received over a network carrier, but timings of the sidelink and network carriers may be different. Such differences in timings of the sidelink and network carriers may create ambiguity regarding the grant for the sidelink carrier.